Which Hemingway story references the running of the bulls" in Spain?"
Chapter 15 of Hemingway's The Sun Also Rises features Spain's seven-day fiesta of San Fermin, during which Jake sleeps while Bill, Cohn, Mike, and Brett stay out drinking all night and then attend the running of the bulls from the corrals to the bullring, through the streets of Pamplona.
Bulls and bullfighting are prominent in The Sun Also Rises. Jake, Bill, Cohn, Mike, and Brett all attend a bullfight, where Cohn grows very upset by the gore while Brett, by contrast, doesn't flinch. Despite Brett's unconventional lifestyle and refusal to believe in a Christian God, she cannot look away from the bullfight because she does not fear death — she may, in fact, be attracted to it. Brett's affair with Romero, the bullfighter, also changes the course of the plot and the lives of the novel's characters.
Many people falsely believe that Hemmingway's Death in the Afternoon is also about the running of the bulls. In fact, this is a nonfiction piece about the history and tradition of bullfighting in general. Death in the Afternoon would be a good book to pick up if you're interested in Hemingway's personal views on the sport.